1. Field of the Invention
The present invention relates to an apparatus for checking the roundness of rotating parts, comprising: support means; control means mounted on the support means for supporting and rotating the part to be checked; measuring means adapted to cooperate with the part for providing at least one signal indicative of the radial dimensions of the part, the measuring means including a substantially rigid support device, two elements fixed with respect to the support device and gauging means fixed to the support device, the two fixed elements being adapted to remain into stable contact with the part; and processing means connected to the gauging means for providing an indication of the out-of-roundness of the part.
2. Description of the Prior Art
So far, the roundness of mechanical parts has been checked, almost exclusively, by means of laboratory instruments comprising a high precision spindle for rotating the part, a measuring head fixed with respect to the spindle support for measuring the radial dimensions of the rotating part and an electronic processing and indicating unit for providing indications of the out-of-roundness as well as, possibly, a chart trace of the part.
These laboratory instruments are rather expensive, fragile and slow. Accordingly, they are not suitable for checking, in workshop environment, high volume productions. Moreover, as mentioned before, they require a high precision spindle, because displacements of the spindle axis cause measurement errors.
U.S. Pat. No. 3,274,693, that mentions these laboratory instruments, describes an apparatus more suitable for roundness checking in workshop environment. This apparatus comprises a spindle that may be of limited precision, at least a pair of Vee devices (with different V-angles) adapted to remain into contact with the surface of the rotating part and gauge probes, each coupled to a relevant Vee device and arranged along the bisector of the Vee device for gauging the radial dimensions of the part in different cross-sections. In the above mentioned U.S. patent it is stated that, by using two Vee devices and relevant gauge probes and by suitably choosing the V-angles and a particular processing of the measurement results, it is possible to measure with satisfactory accuracy the out-of-roundness of parts having three to nine lobes.
The apparatus of U.S. Pat. No. 3,274,693 involves several drawbacks. In order to obtain high accuracy it is necessary to provide a plurality of Vee devices and relevant probes. This involves problems with regard to overall dimensions and cost, the difficulty of guaranteing the continuous and stable contact between the Vee devices and the part, the necessity of determining empirical factors indispensable for the processing. Moreover, it is impossible to check, with the same apparatus, the part diameter.
The measurement of the out-of-roundness of parts is particularly advisable for parts machined on centerless grinding machines. These machines normally provide machined parts with constant diameter, i.e. parts with exactly round cross-sections or with an odd number of lobes. However, parts with an odd number of lobes are frequently obtained and, as machining goes on, the out-of-roundness tends to increase. Therefore, it is important to immediately detect these errors for changing and correcting the set-up of the grinding machine and/or the machining cycle.
U.S. Pat. No. 4,055,027 discloses a gauging head having two feelers for checking the diameter of parts machined on a centerless grinder. The head is arranged so as to check the diameter of the part. At least in theory, one could think to use the head also for checking out-of-roundness errors due to lobing of even order (however, as already mentioned, these errors do not occur in centerless grinders), but the arrangement of the head, that in measuring position is fixed with respect to the grinding machine, makes checking of errors due to lobing of odd order impossible.